Headgear having an air purifier

ABSTRACT

A headgear is described that includes an air purifier, a microphone, and a control unit. The control unit analyses a signal output by the microphone to determine a magnitude of wind. The control unit then controls a flow rate of the air purifier in response to the determined magnitude.

FIELD OF THE INVENTION

The present invention relates to a headgear having an air purifier.

BACKGROUND OF THE INVENTION

Pollutants in the air can be harmful to human health. Air purificationdevices are known that remove pollutants from the air and direct astream of purified air towards the mouth and nose of the wearer. Apotential problem with such devices is that, when worn outdoors, windmay push the stream of purified air away from the mouth and nose of thewearer.

SUMMARY OF THE INVENTION

The present invention provides a headgear comprising an air purifier, amicrophone, and a control unit, wherein the control unit analyses asignal output by the microphone to determine a magnitude of wind, andthe control unit controls a flow rate of the air purifier in response tothe determined magnitude.

With the headgear of the present invention, the flow rate of the airpurifier is controlled in response to a magnitude of wind. The flow ratemay therefore be increased in response to an increase in the magnitudeof wind. As a result, a stronger stream of purified air is generated andthus deviations in the direction of the stream due to the wind may bereduced.

The control unit determines the magnitude of wind based on a signaloutput by a microphone. Microphones typically sense air disturbances inthe range of hundreds of μPa up to tens of Pa. However, even relativelyweak wind can generate pressures that is a hundred times greater thanthis. The headgear therefore exploits these characteristics to provide arelatively cost-effective solution for detecting wind.

The control unit may transform time samples (i.e. time-domain samples)of the signal into one or more frequency samples (i.e. frequency-domainsamples), and determine the magnitude of wind based on energies of thefrequency samples. When air particles hit the diaphragm of themicrophone, they do so in an unpredictable way. Nevertheless, the windhas a recognisable shape in the frequency domain. Accordingly, bytransforming samples of the signal from the time domain to the frequencydomain, and then analysing the energy of the frequency samples, themagnitude of wind may be determined.

The control unit may determine the magnitude of wind based on variationsin the energies with time. Some real-life noises may have energies atlower frequencies, and may therefore be mistaken for wind. The energyassociated with wind may vary significantly with time. By contrast, theenergy associated with a real-life noise may vary comparatively littleover the same time period. Accordingly, the magnitude of wind may bedetermined by analysing temporal variations in the energies of thefrequency samples.

The control unit may determine the magnitude of wind based on at leastone measure comprising a total energy of the frequency samples, and atemporal variance of the total energy. The frequency samples may beselected such that the majority of the energy of wind is containedwithin the frequency samples. Accordingly, by measuring the total energyof the samples, the magnitude of wind may be determined. As alreadynoted, the energy associated with wind may vary significantly with time,whereas the energy associated with real-life noises may varycomparatively little. Accordingly, the magnitude of wind may determined,additionally or alternatively, by measuring the temporal variance of thetotal energy. The control unit may determine the magnitude of wind basedon both the total energy of the frequency samples and the temporalvariance of the total energy. As a result, a more reliable determinationmay be made.

The control unit may compare the measure against one or more thresholdsand determine the magnitude of wind based on the comparison. Forexample, the control unit may determine that the magnitude of wind islow when the measure is less than a threshold and high when the measureis greater than the threshold. As a result, the magnitude of wind may bedetermined in relatively simple manner that can be implementedcost-effectively in hardware.

The frequency samples may have a maximum frequency no greater than 50Hz. Changes in the energy of the signal due to wind occur predominantlyat low frequencies. More particularly, the majority of the energy iscontained at frequencies below about 500 Hz. Many real-life noises mayhave energies at these frequencies. However, very few real-life noiseshave significant energies at frequencies below 50 Hz. Accordingly, byemploying a maximum frequency of 50 Hz for the samples, the magnitude ofwind may be determined more reliably with fewer false triggers.

The headgear may comprise a speaker and an active noise cancellationunit, and the microphone may be a microphone of the active noisecancellation unit. As a result, a cost-effective solution is providedfor controlling the flow rate of the air purifier in response to wind.In particular, a single microphone may be used for two very differentpurposes.

The headgear may comprise a further microphone, and the control unit mayanalyse the signal output by the microphone and a further signal outputby the further microphone to determine the magnitude of wind. By usingtwo microphones, a more reliable determination of the magnitude of windmay be made.

The control unit may transform time samples of the signal into one ormore frequency samples, and time samples of the further signal into oneor more further frequency samples. The control unit may then determinethe magnitude of wind based on energies of the frequency samples and thefurther frequency samples. Wind has a recognisable shape in thefrequency domain. Accordingly, by transforming samples of the signal andfurther signal from the time domain to the frequency domain, and thenanalysing the energy of the resulting samples, the magnitude of wind maybe determined.

The control unit may determine the magnitude of wind based ondifferences in the energies of the frequency samples and the furtherfrequency samples. At relatively low frequencies, where the majority ofthe energy from wind is contained, real-life noises will have arelatively long wavelength. Accordingly, real-life noises are likely tohave similar energy signatures in the signals of the microphone and thefurther microphone. However, when wind particles impact the diaphragmsof the two microphones, they may do so in a random way that is unique toeach microphone. As a result, wind is likely to manifest itself asdifferent energies in the signals of the microphone and the furthermicrophone. Accordingly, by analysing differences in the frequencysamples of the two signals, the magnitude of wind may be determined morereliably with fewer false triggers.

The control unit may determine the magnitude of wind based on variationsin the differences with time. The energies associated with windtypically vary with time. By contrast, the energies associated withreal-life noises, particularly at relatively low frequencies, may varycomparatively little over the same time period. Accordingly, by not onlyanalysing the differences in the energies of the two signals but alsohow those differences vary with time, a more reliable determination ofthe magnitude of wind may be made.

The control unit may determine a coherence of the signal and the furthersignal, and determine the magnitude of wind based on the coherence.Coherence is a measure of the relationship between the two microphonesignals, and may therefore be used to evaluate similarity. As noted, atrelatively low frequencies, where the majority of the energy from windis contained, real-life noises will have a relatively long wavelength.Accordingly, real-life noise is likely to have a similar energysignature (albeit at potentially different amplitudes) in each of themicrophone signals. By contrast, wind is likely to have different energysignatures in the two microphone signals. Accordingly, the coherence ofthe two signals may provide a relatively good measure of the magnitudeof wind.

The control unit may determine the magnitude of wind based on at leasttwo of: the energies of the samples and/or the further samples;variations in the energies of the samples and/or further samples withtime; differences in the energies of the samples and the furthersamples; and variations in the differences in the energies of thesamples and the further samples. By using at least two differentmeasures, a more reliable determination of the magnitude of wind may bemade.

The headgear may comprise a speaker and an active noise cancellationunit. The microphone may be a feedforward microphone of the active noisecancellation unit and the further microphone may be a feedbackmicrophone of the active noise cancellation unit. As a result, acost-effective solution is provided for controlling the flow rate of theair purifier in response to wind. In particular, the two microphones maybe used for two very different purposes. This arrangement has thefurther advantage that the feedback microphone is isolated or shieldedfrom the wind. As a result, incoherence or other differences in the twosignals of the microphones due to wind will be amplified and thus a morereliable determination may be made.

The headgear may comprise a left ear cup and a right ear cup. Themicrophone may be provided in the left ear cup, and the furthermicrophone may be provided in the right ear cup. This then has theadvantage that, in addition to determining the magnitude of wind, thecontrol unit may also determine the direction of wind.

Each ear cup may comprise a speaker and an active noise cancellationunit. The microphone may be a feedforward microphone of the active noisecancellation unit of the left ear cup, and the further microphone may bea feedforward microphone of the active noise cancellation of the rightear cup. As a result, a cost-effective solution is provided forcontrolling the flow rate of the air purifier in response to wind. Inparticular, the microphones may be used for two very different purposes.This arrangement has the further advantage that both microphones areexposed and are thus sensitive to the wind.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example, with reference tothe accompanying drawings in which:

FIG. 1 illustrates a headgear in accordance with an embodiment;

FIG. 2 is a simplified illustration of a section through the headgear;

FIG. 3 illustrates an ear cup of the headgear;

FIG. 4 is a section through the ear cup;

FIG. 5 illustrates a nozzle of the headgear;

FIG. 6 is a block diagram of components of the headgear;

FIG. 7 is a block diagram of a wind detect module of the headgear; and

FIG. 8 illustrates the frequency response of five microphones, only oneof which (indicated by an arrow) was exposed to wind.

DETAILED DESCRIPTION OF THE INVENTION

The headgear 1 of FIGS. 1 to 6 comprises a headband 2, a left ear cup 3,a right ear cup 4 and a nozzle 5.

The headband 2 is attached at one end to the left ear cup 3, and at theopposite end to the right ear cup 4. The headband 2 house one or morebatteries 6 for powering electrical components of the ear cups 3,4.

Each ear cup 3,4 comprises a housing 10, a speaker assembly 11, an airpurifier 12, and an ear pad 13. Additionally, one of the ear cups 3,4comprises a control unit 14.

The housing 10 houses the speaker assembly 11, the air purifier 12 and(for one of the ear cups) the control unit 14, and comprises an airinlet 20 and an air outlet 21. The air inlet 20 comprises a plurality ofapertures in a wall of the housing 10. The air outlet 21 is provided atthe end of an outlet duct 22 of the housing 10.

The speaker assembly 11 comprises a speaker 25 and an active noisecancellation (ANC) unit 26. The ANC unit 26 comprises a feedforwardmicrophone 27, a feedback microphone 28 and an ANC circuit 29. The ANCcircuit 29 is coupled to the feedforward 27 and feedback microphones 28,and to the speaker 25. In response to signals received from thefeedforward and feedback microphones 27,28, the ANC circuit 29 generatesan output signal for driving the speaker 25.

The air purifier 12 comprises an electric motor 30, an impeller 31 and afilter 32. The impeller 31 is driven by the electric motor 30 and, whendriven, causes air to drawn in through the air inlet 20 of the housing10. The air is drawn through the filter 32, which is located upstream ofthe impeller 31. The air is purified by the filter 32, and the purifiedair is discharged via the air outlet 21 of the housing 10.

The control unit 14 comprises a wind detect module 35 and a motorcontrol module 36.

The wind detect module 35 is coupled to the feedforward and feedbackmicrophones 27,28 of both ear cups 3,4. The wind detect module 35analyses the signals output by the microphones 27,28 to determine amagnitude and/or a direction of wind.

The motor control module 36 controls the electric motor 30 of each earcup 3,4. More specifically, the motor control module 36 generates drivesignals (e.g. PWM signals) for controlling the speeds of the electricmotors 30 and thus the flow rates of the air purifiers 12. The motorcontrol module 36 is coupled to the wind detect module 35. In responseto the magnitude and/or direction of the wind determined by the winddetect module 35, the motor control module 36 controls the flow rates ofthe air purifiers 12.

The nozzle 5 is releasably attached to the left and right ear cups 3,4.More specifically, the nozzle 5 is releasably attached to the outletducts 22 of the left and right ear cups 3,4. The nozzle 5 comprises acurved duct 40 having a first inlet 41 located at one end of the duct40, a second inlet 42 located at an opposite end of the duct 40, and anoutlet 43 located midway along the length of the duct 40. The outlet 43comprises an aperture in the duct 40 covered by mesh. When attached tothe ear cups 3,4, the first inlet 41 of the nozzle 5 receives a firstairflow from the air purifier 12 of the left ear cup 3, and the secondinlet 42 receives a second airflow from the air purifier 12 of the rightear cup 4. The two airflows travel within the duct 40 and combine at theoutlet 43. The combined airflow is then discharged from the nozzle 5 viathe outlet 43.

When the headgear 1 is worn by a wearer, the combined airflow of the twoair purifiers 12 is discharged as a stream of purified air towards themouth and nose of the wearer. When the headgear 1 is worn outdoors, windmay push the stream of purified air away from the mouth and nose of thewearer. In order to compensate for this, the control unit 14 controlsthe flow rates of the air purifiers 12 in response to changes in thewind.

The wind detect module 35 analyses the signals output by the microphones27,28 of the headgear 1 and, in response, determines a magnitude and/ora direction of the wind. The analysis performed by the wind detectmodule 35 is described below in more detail. In response to thedetermined magnitude and/or direction, the motor control unit 36controls the flow rate of the air purifiers 12.

In a first example, the wind detect module 36 may determine a magnitudeof wind. More particularly, the wind detect module 35 may determine thatthe magnitude of the wind is either low or high. When the magnitude ofthe wind is low, the motor control unit 36 drives the electric motors 30of the air purifiers 12 at a first speed such that each air purifier 12generates purified air at a first flow rate. The airflows of the two airpurifiers 12 combine at the outlet 43 of the nozzle 5 to generate astream of purified air that is directed towards the mouth and nose ofthe wearer at a first velocity. When the wind detect module 35determines that the magnitude of wind is high, the motor control unit 36drives the electric motors 30 of the air purifiers 12 at a second,higher speed such that each air purifier 12 generates purified air at asecond, higher flow rate. As a result, a stream of purified air isdirected towards the mouth and nose of the wearer at a second, highervelocity. Consequently, in response to an increase in the magnitude ofthe wind, the velocity of the stream of purified air is increased.Deviations in the direction of the stream due to the wind are thusreduced, and therefore purified air continues to be maintained at themouth and nose of the wearer.

In a second example, the wind detect module 35 may determine a directionof wind. More particularly, the wind detect module 35 may determine thatthe direction of the wind is from the left, from the right, or from thefront/rear relative to the headgear 1.

When the direction of wind is from the left, the motor control unit 36drives the electric motor 30 of the air purifier 12 of the right ear cup4 at a higher speed than that of the left ear cup 3. This may beachieved by increasing the speed of the electric motor 30 of the rightear cup 4 and/or by decreasing the speed of the electric motor 30 of theleft ear cup 3. As a result of the different speeds, the air purifier 12of the right ear cup 4 generates purified air at a higher flow rate thanthat of the air purifier 12 of the left ear cup 3. The two airflowscontinue to combine at the outlet 43 of the nozzle 5. However, becausethe two airflows have different flow rates, the stream of purified airdischarged from the outlet 43 is no longer directed straight ahead butis instead skewed to one side. In this particular instance, the airpurifier 12 of the right ear cup 4 generates a higher flow rate. As aresult, the stream of purified air is skewed to the left. The stream ofpurified air is therefore skewed in a direction that opposes the wind.The resultant stream of purified air (i.e. the resultant of the streamdischarged from the nozzle and the wind) arrives at the mouth and noseof the wearer.

When the direction of wind is from the right, the motor control unit 36drives the electric motor 30 of the left ear cup 3 at a higher relativespeed. As a result, the air purifier 12 of the left ear cup 3 generatesa higher flow rate and thus the stream of purified air is skewed to theright. When the direction of wind is from the front or rear, the motorcontrol unit 36 drives the electric motors 30 of both air purifiers 12at the same speed. As a result, the air purifiers 12 generate purifiedair at the same flow rates and thus the stream of purified air isdirected straight ahead.

The control unit 14 therefore controls the relative flow rates of theair purifiers 12 in response to the determined direction of the wind.More specifically, in response to determining that the direction of windis from a side of the headgear 1, the control unit 14 increases therelative flow rate of the air purifier 12 located on the downstream sideof the headgear 1. As a result, a stream of purified air is dischargedfrom the nozzle 5 in a direction that opposes the wind, and thus theresultant stream of purified air arrives at the mouth and nose of thewearer.

In the first example described above, the wind detect module 35determines whether the magnitude of wind is low or high. It will beappreciated that the wind detect module 35 may use other scales whendetermining the magnitude of wind. For example, the wind detect module35 may determine that the magnitude of wind has a value of between 0 and10, where 0 is no wind and 10 is high wind. Similarly, in the secondexample, the wind detect module 35 determines whether the magnitude ofwind is from the left, the right, or the front/back. Again, the winddetect module 35 may use other scales when determining the direction ofwind. For example, the wind detect module 35 may determine that thedirection of wind has a value of between −10 to +10, where −10 is acrosswind directly from the left, +10 is a crosswind directly from theright, and 0 is a headwind or tailwind.

The wind detect module 35 may determine both the magnitude of wind andthe direction of wind. In this instance, the motor control unit 36controls the relative flow rates of the air purifiers 12 in response toboth the magnitude and the direction of the wind.

Referring now to FIG. 7 , the wind detect module 35 comprises ananalogue-to-digital converter (ADC) unit 37, a spectrum analyser 38, anda wind determiner unit 39. The ADC 37 unit converts the signals of thefour microphones 27,28 from analogue to digital. The spectrum analyser38 transforms each of the digital microphone signals from the timedomain to the frequency domain. The spectrum analyser 38 uses a fastFourier transform (FFT) or other discrete Fourier transform in order totransform time-domain samples of the microphone signal intofrequency-domain samples (sometimes referred to as bins). Each frequencysample represents the amount of energy that the microphone signal has atthat particular frequency. The wind determiner unit 39 analyses theenergies of the frequency samples and, in response, determines amagnitude of wind and/or a direction of wind.

The microphones 27,28 of the headgear 1 are designed to sense airdisturbances in the range of hundreds of μPa up to tens of Pa. However,even weak wind (e.g. 1 on the Beaufort scale) can generate pressuresthat a hundred times greater than this. The wind detect module 35therefore uses the microphones 27,28 as sensitive pressure sensors forsensing the magnitude and/or direction of wind.

When air particles hit the diaphragm of a microphone they do so in anunpredictable way. Nevertheless, the wind has a recognisable shape inthe frequency domain. FIG. 8 is a time-averaged plot of the frequencyresponse of five microphones, only one of which (indicated by an arrow)was exposed to wind. The shape or energy of the microphone signal varieswith frequency and depends on, among other things, the position of themicrophone, the housing and surrounding structures of the ear cup, aswell as the magnitude and direction of the wind. Nevertheless, changesin the shape of the signal due to wind occur predominantly at lowfrequencies and usually the majority of the energy is contained atfrequencies below about 500 Hz. The wind detect module 35 exploits thisbehaviour in order to determine the magnitude and/or direction of thewind.

As described below, there are various methods which the wind detectmodule 35 may employ to determine the magnitude and/or direction of thewind. Although the headgear 1 comprises four microphones (twomicrophones 27,28 in each ear cup 3,4), some of the methods employed bythe wind detect module 35 may be implemented using fewer microphones.Indeed, some of the methods may be implemented using just onemicrophone.

In each of the methods described below, the wind detect module 35analyses the microphone signals and determines a magnitude and/ordirection of wind based on the energies of the signals over a predefinedfrequency range. As noted above, the majority of the energy of the windis contained at frequencies below about 500 Hz. Many real-life noisesmay have energies at these frequencies. However, very few real-lifenoises have significant energies at frequencies below about 50 Hz.Accordingly, the predefined frequency range employed by the wind detectmodule 35 may be, for example, 0 to 50 Hz. As a result, the magnitudeand/or direction of wind may be determined more reliably with fewerfalse triggers.

The spectral analyser 38 may use a sampling frequency such that a singlefrequency sample is generated that spans the predefined frequency range.Alternatively, the spectral analyser 38 may use a sampling frequencysuch that a plurality of frequency samples are generated that span thepredefined frequency range. The spectral analyser 38 may therefore besaid to generate one or more frequency samples that span the predefinedfrequency range.

In a first method, the wind detect module 35 determines a magnitude ofwind using just one of the feedforward microphones 27.

The wind determiner unit 39 determines the magnitude of wind based onthe total energy of the one or more frequency samples. Moreparticularly, the wind determiner unit 39 compares the total energy ofthe samples against one or more thresholds, and determines the magnitudeof wind based on the comparison. For example, the wind determiner unit39 may compare the total energy of the samples against a singlethreshold. The wind determiner unit 39 then determines that themagnitude of wind is low if the total energy is less than the threshold,and high if the total energy is greater than the threshold.

The wind determiner may compare the total energy of different frequencysamples against different thresholds. For example, the wind determinerunit 39 may determine that the magnitude of wind is high only when thetotal energy of a first sample(s) is greater than a first threshold andthe total energy of a second sample(s) is greater than a second,different threshold.

The energy signature or shape of the wind may vary significantly withtime. The time resolution of the spectral analyser 38 may therefore bedefined so as to smooth out these short-term variations. Alternatively,the wind determiner unit 39 may determine the magnitude of wind based onthe total energy of frequency samples at different time intervals. Forexample, the spectral analyser 38 may generate a first set of frequencysamples at time T1, and a second set of frequency samples at time T2.The wind determiner unit 39 then sums or averages the energies of bothsets of samples in order to determine the total energy.

A potential problem with the first method is that some real-life noises(e.g. thunder, surf, an overhead helicopter) may have energies containedwithin the predefined frequency range, and thus be mistaken for wind.

In a second method, the wind detect module 35 again determines amagnitude of wind using just one of the feedforward microphones 27.However, rather than determining the magnitude of wind based on thetotal energy of the frequency samples, the wind determiner unit 39instead determines the magnitude of wind based on variations in thetotal energy with time.

As already noted, the energy signature of wind may vary significantlywith time. By contrast, the energy signature of real-life noise (atthese low frequencies) may vary comparatively little over the sametimescale. Accordingly, the wind determiner unit 39 determines themagnitude of wind based on temporal variations in the total energy ofthe frequency samples.

The wind determiner unit 39 determines differences in the total energyof the samples at different time intervals. For example, the spectralanalyser 38 may generate a first set of samples at time T1, and a secondset of samples at time T2. The wind determiner unit 39 then determinesdifferences in the energies of the first and second set of samples, anddetermines the magnitude of wind based on these differences.

The wind determiner unit 39 may determine a measure representative ofthe temporal variance of the total energy of the samples. For example,the wind determiner unit 39 may determine the sum of squared differencesor the sum of absolute differences. The wind determiner unit 39 thencompares the measure (e.g. sum of squares) against one or morethresholds in order to determine the magnitude of the wind. For example,the wind determiner unit 39 may determine that the magnitude of wind islow if the measure is less than a threshold, and high if the measure isgreater than the threshold.

The wind detect module 35 may employ both the first method and thesecond method in order to determine more reliably the magnitude of wind.In this instance, the wind determiner unit 39 determines the magnitudeof wind based on both the total energy of the samples and also temporalvariations in the total energy. So, for example, the wind determinerunit 39 may determine that the wind is high only if the total energy ofthe samples is greater than a first threshold and the sum of squares ofthe differences in the total energies is greater than a secondthreshold.

In employing both the first method and the second method, the winddetect module 35 provides a more reliable determination of the magnitudeof wind. Nevertheless, real-life noises having energies within thepredefined frequency range may be short-lived, and thus be mistaken forwind.

In a third method, the wind detect module 35 determines a magnitude ofwind using the feedforward microphones 27 of both ear cups 3,4.

At relatively low frequencies, where the majority of the energy fromwind is contained, real-life noises will have a relatively longwavelength and are not therefore significantly altered by the headgear 1or the human body. Accordingly, over the predefined frequency range(e.g. below 50 Hz), the two feedforward microphones 27 will detectreal-life noises at similar energies and phases. However, when windparticles impact the diaphragms of the two microphones 27, they do so ina random way that is unique to each microphone. As a result, the windmanifests itself as different energies in the signals of the twomicrophones 27. The wind detect module 35 therefore exploits thisbehaviour in order to determine the magnitude of the wind.

The wind detect module 35 determines the magnitude of the wind based ona comparison of the two microphone signals. More particularly, the winddeterminer unit 39 determines a magnitude of wind based on differencesin the energies of the two microphone signals.

The wind determiner unit 39 may determine the magnitude of wind based ondifferences in the total energies of the frequency samples of the twosignals. For example, the wind determiner unit 39 may determine that themagnitude of wind is low if a measure of the differences (e.g. sum ofsquares or sum of absolutes) is less than a threshold, and high if themeasure is greater than a threshold. Alternatively or additionally, thewind determiner unit 39 may determine the magnitude of wind based ontemporal variations in the differences in the energies of the twosignals. For example, the spectral analyser 38 may generate a first setof samples (for both microphones) at time T1, and a second set ofsamples (again, for both microphones) at time T2. The wind determinerunit 39 may then determine a first difference value (e.g. sum of squaresor sum of absolutes) based on differences in the energies of the firstset of samples, and a second difference value based on differences inthe energies of the second set of samples. The wind determiner unit 39may then determine that the magnitude of wind is high only if both thefirst difference value and the second difference value are greater thana threshold.

The wind detect module 35 may use the third method together with one orboth of the first method and the second method. For example, the winddeterminer unit 39 may determine that the magnitude of wind is high onlyif (i) the total energy of the samples of one of the microphone signalsis greater than a threshold (first method) and (ii) the difference inthe total energies of the two microphone signals is greater than afurther threshold (third method). In this way, the wind determiner unit39 determines that the magnitude of wind is high only if (i) thelow-frequency energy in at least one of the microphone signals is highand (ii) the low-frequency energies of the two microphone signals aresufficiently different. As a further example, the wind determiner unit39 may determine that the magnitude of wind is high only if (i) thedifference in the total energies of one of the microphone signals over agiven time period is greater than a threshold (second method) and (ii)the differences in the total energies of the two microphone signals overthe same time period is greater than a further threshold (third method).In this way, the wind determiner unit 39 determines that the magnitudeof wind is high only if (i) the low-frequency energy in at least one ofthe microphone signals varies with time, and (ii) the low-frequencyenergies of the two microphone signals are sufficiently different atdifferent times.

In a fourth method, the wind detect module 35 determines a magnitude ofwind using two microphones. The first microphone is the feedforwardmicrophone 27 of one of the ear cups, and the second microphone iseither the feedback microphone 28 of the same ear cup or the feedforwardmicrophone 27 of the opposite ear cup.

The wind determiner unit 39 determines a magnitude of wind based on thecoherence of the two microphone signals. Coherence is a measure of therelationship between the two microphone signals, and may therefore beused to evaluate similarity. Any noise present in one of the microphonesignals but not the other will result in a lower coherence value. Fortwo microphones located in relatively close proximity, real-life noisewill have a similar energy signature (albeit at potentially differentamplitudes) in each of the microphone signals, at least at these lowfrequencies. By contrast, wind will have very different energies in thetwo microphone signals. Accordingly, the coherence of the two signalsmay be used to determine the magnitude of wind. For example, the winddeterminer unit 39 may determine that the magnitude of wind is low ifthe coherence is greater than a threshold (i.e. the two signals aresimilar) and high if the coherence is less than the threshold (i.e. thetwo signals are dissimilar).

Again, the wind detect module 35 may use the fourth method together withone or more of the other methods. For example, the wind determiner unit39 may determine that the wind is high only if (i) the total energy ofat least one of the microphone signals is greater than a threshold(first method), and (ii) the coherence of the two microphones signals isless than a further threshold (fourth method).

The first microphone may be a feedforward microphone 27 and the secondmicrophone may be a feedback microphone 28. This arrangement has theadvantage that the two microphones 27,28 are located in close proximity,and thus real-life noise will result in substantially the same energysignature for both microphones, at low frequencies. Moreover, thefeedback microphone 28 is isolated or shielded from the wind. As aresult, incoherence in the two signals due to wind will be significantlyincreased. However, a potential disadvantage with this arrangement isthat the speaker 25 of the ear cup 3,4 may generate sounds (e.g.sub-bass sounds) having energies within the predefined frequency range.As a result, the incoherence of the two signals will increase.

The first microphone may be a feedforward microphone 27 of one ear cup3, and the second microphone may be a feedforward microphone 27 of theopposite ear cup 4. This arrangement then has the advantage that bothmicrophones 27 are exposed to the wind. However, the microphones 27 arepositioned further apart and thus differences in the two signals due toreal-life noise will increase. Additionally, should the wearer grasp andmanipulate one of the ear cups, the resulting noise will increase theincoherence of the two signals and may therefore be interpreted as wind.Furthermore, the sound generated by the air purifier 12 in the left earcup 3 may differ from that generated by the air purifier 12 in the rightear cup 4, which again will increase the incoherence in the two signals.

So far reference has been made to determining a magnitude of wind.However, the wind detect module 35 may additionally or alternativelydetermine a direction of wind.

In a fifth method, the wind detect module determines a direction of windusing the two feedforward microphones 27.

The fifth method is essentially an expansion of the first method. Thewind determiner unit 29 determines the total energy of the firstmicrophone (e.g. left ear cup) and the total energy of the secondmicrophone (e.g. second ear cup). The wind determiner unit 39 thendetermines a direction of wind based on a comparison of the twoenergies. For example, the wind determiner unit 39 may determine thatthe wind is coming from the left if the total energy of the firstmicrophone is greater, and from the right if the total energy of thesecond microphone is greater. The wind determiner unit 39 thendetermines that the wind is coming from the front or rear if the totalenergy of the two microphones are the same or similar. In a furtherexample, the wind determiner unit 39 may determine that the wind is acrosswind if the difference in the total energies of the two signals isgreater than a threshold, and a headwind or tailwind if the differenceis less than the threshold.

The wind detect module 35 may combine the fifth method with one or moreof the previously described methods in order better determine thedirection of wind. For example, the total energy of the first microphonemight be greater than that of the second microphone, suggesting that thewind is coming from the left. However, the energy of the firstmicrophone may be relatively constant with time (indicative of real-lifenoise), whereas the energy of the second microphone may be variable(indicative of wind). The wind determiner unit 39 may thereforedetermine the direction of wind based on (i) the total energies of thetwo microphone signals (fifth method) and (ii) temporal variations inthe energies of the two microphone signals (third method). As a result,the wind detect module 35 may make a more reliable determination of thedirection of wind.

In a sixth method, the wind detect module 35 determines a direction ofwind using the feedforward and feedback microphones 27,28 of both earcups 3,4.

The wind determiner unit 39 determines a magnitude of wind at each earcup 3,4 based on the coherence of the signals of the feedforward andfeedback microphones for that ear cup. The wind determiner unit 39 mayadditionally use one or more of the other methods described above todetermine the magnitude of wind at each ear cup 3,4. The wind determinerunit 39 then determines the direction of wind based on a comparison ofthe magnitudes of wind. So, for example, the wind determiner unit 39 maydetermine that the wind is coming from the left if the magnitude of windat the left ear cup 3 is greater, from the right if the magnitude ofwind at the right ear cup 4 is greater, and from the front or rear ifthe magnitudes of wind for the two ear cups 3,4 are the same or similar.

It will be apparent from the above that the wind detect module 35 mayemploy different methods and/or permutations of methods in order todetermine the magnitude and/or direction of wind. In the example methodsdescribed above, the wind detect module 35 determines whether themagnitude of wind is low or high, and/or whether the direction of windis from the left, right or front/rear. However, as already noted, thewind detect module 35 may use other scales when determining themagnitude and/or direction of wind. This may be achieved, for example,through the use of multiple thresholds.

The headgear 1 has four microphones 27,28. However, as described above,the wind detect module 35 is capable of determining the magnitude and/ordirection of wind using a fewer number of microphones. In particular,the wind detect module 35 is capable of determining the magnitude ofwind using just one microphone, and the direction of wind using just twomicrophones.

The wind detect module 35 makes use of the ANC microphones 27,28 of theheadgear 1. This then provides a cost-effective solution for controllingthe flow rates of the air purifiers 12 in response to changes in wind.However, the headgear 1 may comprise additional or alternativemicrophones, which the wind detect module 35 may use to determine themagnitude and/or direction of wind. For example, the headgear 1 maycomprise one or more telephony microphones. In particular, the headgear1 may comprise a pair of telephony microphones on one or both of the earcups 3,4. Pairs of telephony microphones may be placed in closeproximity to one another to provide beamforming. As a result, themicrophones, both of which are exposed to wind, are well-suited atdetecting wind.

The headgear 1 comprises a pair of air purifiers 12. This arrangementhas several advantages over say a single air purifier. For example, theweight of the headgear 1 is better balanced between the two ear cups3,4. Additionally, a stream of purified air may be generated at a givenflow rate by driving the electric motors 30 at lower speeds, which inturn reduces noise. Nevertheless, in spite of these advantages, theheadgear 1 could conceivably comprise a single air purifier. The motorcontrol unit 36 would continue to control the flow rate of the airpurifier in response to changes in the magnitude of the wind. Inresponse to changes in the direction of the wind, the headgear 1 mightinclude a butterfly valve or other means located at the outlet 43 of thenozzle 5, which is moved in order to change the direction in which thestream of purified air is discharged.

Whilst particular embodiments have thus far been described, it will beunderstood that various modifications may be made without departing fromthe scope of the invention as defined by the claims.

1. A headgear comprising an air purifier, a microphone, and a controlunit, wherein the control unit analyses a signal output by themicrophone to determine a magnitude of wind, and the control unitcontrols a flow rate of the air purifier in response to the determinedmagnitude.
 2. The headgear as claimed in claim 1, wherein the controlunit transforms time samples of the signal into one or more frequencysamples, and determines the magnitude of wind based on energies of thefrequency samples.
 3. The headgear as claimed in claim 2, wherein thecontrol unit determines the magnitude of wind based on variations in theenergies with time.
 4. The headgear as claimed in claim 2, wherein thecontrol unit determines the magnitude of wind based on at least onemeasure comprising a total energy of the frequency samples, and atemporal variance of the total energy.
 5. The headgear as claimed inclaim 4, wherein the control unit compares the measure against one ormore thresholds and determines the magnitude of wind based on thecomparison.
 6. The headgear as claimed in claim 2, wherein the frequencysamples have a maximum frequency no greater than 50 Hz.
 7. The headgearas claimed in claim 1, wherein the headgear comprises a speaker and anactive noise cancellation unit, and the microphone is a microphone ofthe active noise cancellation unit.
 8. The headgear as claimed in claim1, wherein the headgear comprises a further microphone, and the controlunit analyses the signal output by the microphone and a further signaloutput by the further microphone to determine the magnitude of wind. 9.The headgear as claimed in claim 8, wherein the control unit transformstime samples of the signal into one or more frequency samples,transforms time samples of the further signal into one or more furtherfrequency samples, and determines the magnitude of wind based onenergies of the frequency samples and the further frequency samples. 10.The headgear as claimed in claim 9, wherein the control unit determinesthe magnitude of wind based on differences in the energies of thefrequency samples and the further frequency samples.
 11. The headgear asclaimed in claim 10, wherein the control unit determines the magnitudeof wind based on variations in the differences with time.
 12. Theheadgear as claimed in claim 8, wherein the control unit determines acoherence of the signal and the further signal, and determines themagnitude of wind based on the coherence.
 13. The headgear as claimed inclaim 8, wherein the control unit determines the magnitude of wind basedon at least two of: the energies of the samples and/or the furthersamples; variations in the energies of the samples and/or furthersamples with time; differences in the energies of the samples and thefurther samples; and variations in the differences in the energies ofthe samples and the further samples.
 14. The headgear as claimed inclaim 8, wherein the headgear comprises a speaker and an active noisecancellation unit, the microphone is a feedforward microphone of theactive noise cancellation unit and the further microphone is a feedbackmicrophone of the active noise cancellation unit.
 15. The headgear asclaimed in claim 8, wherein the headgear comprises a left ear cup and aright ear cup, the microphone is provided in the left ear cup, and thefurther microphone is provided in the right ear cup.
 16. The headgear asclaimed in claim 15, wherein each ear cup comprises a speaker and anactive noise cancellation unit, the microphone is a feedforwardmicrophone of the active noise cancellation unit of the left ear cup,and the further microphone is a feedforward microphone of the activenoise cancellation of the right ear cup.